Atrial fibrillation (“AF”) of the human heart is a common arrhythmia which is estimated to affect anywhere from 2.2 million to about 5.1 million Americans, as well as approximately 5% of the elderly population over 69 years of age. Theoretically, the AF mechanism involves two main processes: (1) higher automaticity in one or more rapidly depolarizing foci and (2) reentry of conduction involving one or more circuits. Rapid atrial foci, often located in at least one of the superior pulmonary veins, can begin AF in predisposed patients. In addition, the “multiple-wavelet hypothesis” has been proposed as a potential mechanism for AF caused by conduction reentry. According to the hypothesis, normal conduction wave fronts break up, resulting in a number of self-perpetuating “daughter” wavelets that spread through the atria causing abnormal contraction of the myocardium.
Surgical treatment of AF requires the construction of barriers to conduction within the right atrium and left atrium to restrict the amount of myocardium available to spread reentrant wave fronts, thereby inhibiting sustained AF. By making incisions in the myocardium, conduction is interrupted. Since it has been demonstrated that the pulmonary veins often contain the specific rapidly-depolarizing loci, incisions encircling the pulmonary veins can help prevent AF. Similarly, potentially arrhythmogenic foci close to the pulmonary veins, as well as specific atrial regions with the shortest refractory periods, may be isolated from the rest of the atria by strategically placed incisions. Although the risk of such surgery alone is typically less than 1%, the need for median sternotomy and the use of cardiopulmonary bypass, as well as a risk of short-term fluid retention, make this procedure less than ideal.
As an alternative to surgery, catheter ablation has evolved as a standard therapy for patients at high risk for ventricular and supraventricular tachyarrhythmia. The recognition that foci triggering AF frequently initiate within the pulmonary veins has led to ablation strategies that target this zone or that electrically isolate the pulmonary veins from the left atrium. In the superior vena cava, the right atrium, left atrium, and coronary sinus were found as other sites of arrhythmogenic foci. The frequency of recurrent AF has been reduced in more than 60% of patients by the ablation of the foci (superior vena cava, the right and left atria, and the coronary sinus). However, the risk of recurrent AF following a focal ablation procedure is still between 30% to 50% over the first year and is even higher when the ablation involves an attempt to isolate more than one pulmonary vein.
In most circumstances, the cardiac ablation catheter is inserted into a blood vessel (artery or vein), usually through an entry site located in the upper leg or neck. Under fluoroscopy, the tube is navigated through the blood vessels until it reaches the heart. In the heart, electrodes at the catheter tip gather data that pinpoint the location of faulty tissue in the heart (electrical mapping). Once the site is identified, the device delivers either radiofrequency energy (RF ablation) or intense cold (cryoablation) to destroy the small section of tissue. The major goal of this procedure is segmental pulmonary vein isolation and circumferential pulmonary vein ablation. The circumferential ablation strategy yields either an atriovenous electrical disconnection, as demonstrated by elimination of pulmonary vein ostial potentials and absence of discrete electrical activity inside the lesion during pacing from outside the ablation line, or a profound atrial electroanatomical remodeling as expressed by voltage abatement inside and around the encircled areas involving to some extent the posterior wall of the left atrium. The endpoint is the electrical isolation of the pulmonary veins from the left atrium, as they house foci triggering AF in about 80% to about 95% of cases and seem to play a key role in arrhythmia maintenance.
Possible complications of catheter ablation for AF include systemic embolism, pulmonary vein stenosis, pericardial effusion, cardiac tamponade, and phrenic nerve paralysis. The majority of these risks stem from the ablation of an incorrect region. Hence, proper navigation during cardiac ablation is one of the greatest challenges for the electrophysiologist performing the procedure.
Visualization of endocardial structure and ablation lesions through flowing blood has been an obstacle for proper navigation during cardiac ablation. Currently, clinicians perform cardiac ablation using intracardiac echo based on ultrasound. A catheter is advanced from the femoral vein into the heart, thereby allowing the clinician to observe the heart from the inside. This method enables good anatomy imaging, and the clinician can view the electrode-tissue interface during the ablation. Despite this technology, however, the clinician cannot have complete certainty after the ablation procedure that the procedure created a permanent lesion that has destroyed only the targeted tissue and nothing more.
Another method used to determine the accuracy of the ablation is to compare the electrical signals in the heart before and after the procedure to determine whether certain arrhythmogenic signals have been eliminated. However, this method does not always provide sufficient evidence that a permanent lesion has been created as a result of the ablation.
Thus, these approaches fall short of providing optimum clarity and accuracy regarding the ablation. Furthermore, conventional technologies do not combine the function of direct visualization and ablation into one catheter, but instead require the use and coordination of multiple catheters, thereby inherently increasing the risks to the patient.
A new technique has emerged that allows an electrophysiologist to create a real-time 3-D electroanatomical cardiac map using GPS-like technology called CARTO™. The created map is then merged with CT or MRI images providing detailed structures of the chambers of the heart. Real-time intracardiac echocardiography, along with fluoroscopy, is also used to enhance the safety and efficacy of the procedure. Another system, called the Localisa® Intracardiac Navigation System, allows a user to continuously monitor mapping and ablation catheter positions, thus facilitating pulmonary vein isolation procedures and reducing radiation exposure to the patient and medical personnel.
Although these newer systems have significant potential, they are generally unavailable to the typical electrophysiology laboratory because of cost. Thus, there is a need for an efficient, easy to use, and reasonably priced technique for localization and ablation that can be adapted for use in virtually any clinic.